1. Field of the Invention
The present invention relates to an anti cross-thread bolt which prevents itself from being cross-threaded even when obliquely screwed into a female screw.
2. Description of Related Art
When a bolt is screwed into a nut or any other female screw, it is preferable to insert the bolt by precisely aligning the bolt axis with the female screw axis. In actual assembly lines or the like, however, there are some cases where bolts are inserted using tools such as power screwdrivers with their axes being somewhat inclined. In such cases, the threads of the bolt male screw part fit into the root parts of the female screw that are deviated by one pitch from the proper root parts, and if the bolt is forcibly screwed into the female screw in this state, cross-threading may occur, leading to a serious trouble that the bolt cannot be unscrewed easily.
Conventionally, there have been various kinds of bolts proposed to prevent cross-threading, even if they are inserted into female screws somewhat obliquely. An example of such a bolt is the one disclosed in Patent Citation 1 by the applicant of this invention, where a pilot portion (which is also called a full dog point or a guide portion) is formed at the tip of the regular thread part and the outer periphery of the tip of the pilot portion is contacted with the female screw when the bolt is obliquely inserted, thereby correcting the posture of the bolt. Further, as disclosed in Patent Citation 2 and Patent Citation 3, there have been anti cross-thread bolts proposed having various kinds of threads formed on the pilot portions to enhance a posture correction effect.
As a matter of course, however, the pilot portion has a smaller diameter than the regular thread part of a bolt, and an incomplete thread part is inevitably formed between the regular thread part and the pilot portion. As a result, cross-threading may occur on the incomplete thread part. Making the pilot portion longer only reduces the obliquity of the bolt and has no specific effect in preventing cross-threading. Accordingly, making the pilot portion longer is not a good idea in terms of cost and resource.